The goal of this application is to investigate the regulation of autoreactive T lymphocytes. We have found that splenic T cells are stimulated to divide by T:T hybridomas specific for the MHC class II antigens of the T cell donor. These T cells appear to respond to idiotypic determinants on the T cell receptor on the T:T hybridomas. The specific aims of this grant are to: 1. establish the molecules on the stimulatory cells that are recognized; 2. demonstrate that T cells which are specific for self MHC class II antigens (Type I autoreactive T cells) also stimulate autologous splenic T cells (Type II autoreactive T cells); 3. investigate the relationship between autoreactive T cells and antigen-specific T cells with high affinity for self MHC class II antigens; and 4. assess the immunoregulatory functions of Type I and Type II autoreactive T cells with respect to self tolerance and the immune response to foreign antigens.